Higgs boson(s) Why do we need them? What do they look like?

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Presentation transcript:

Higgs boson(s) Why do we need them? What do they look like? Have we found them?

γ Quantum Electrodynamics predicts one massless spin-1 gauge boson PHOTON

Quantum Chromodynamics predicts (32-1) = 8 massless spin-1 gauge bosons GLUONS

Quantum Flavour Dynamics predicts (22-1) = 3 massless spin-1 gauge bosons ? W Z

W and Z boson are NOT massless Mass of W bosons… 80 GeV Mass of Z bosons … 91 GeV Weigh more than a copper atom

Massive spin-1 particles have 3 polarisations Helicity = +1 or 0 or -1

Massless spin-1 particles have only 2 polarisations Horizontal or vertical polarised photons Helicity = +1 or -1 only Longitudinal polarisation is lost! W and Z bosons need extra degree of polarisation as massive

Giving mass to the W and Z New field H W or Z W or Z W and Z bosons pick up mass from interaction with new scalar field Pops out of vacuum & modifies propagator

“Higgs” field Field must have non-zero vacuum value everywhere Universe filled with “relativistic ether” of this field Coupling to the field gives mass to W and Z

Symmetry breaking & the Higgs field Require: underlying theory is symmetric Vacuum or ground state has broken symmetry

Magnetic material at high temperatures Symmetric in direction

Magnetic material at low temperature Symmetry broken – special direction

Broken symmetry for a complex field

Higgs field Doublet of Complex scalar field 4 degrees of freedom 3 end up as longitudinal polarisations of W and Z bosons 1 left over – excitation of the field – Higgs Boson Doublet of Complex scalar field

all the quarks and leptons In the Standard Model the SAME Higgs field gives mass to the: W boson Z boson all the quarks and leptons

Higgs couplings to mass V H f

Higgs boson production and decay Blackboard

Accelerator complex @ CERN

LINear ACcelerator

Reconstructing the debris P Reconstructing the debris

Detectors… Robotic assembly of precision silicon tracker – Denys Wilkinson Building

ATLAS Segment of detector

H  Z + Z*  (e+ + e-) + (e+ + e-) candidate

A collision producing 2 high-energy photons Higgs   +  ?

Higgs  2 photons Bump at mass of new particle

Couplings

How the particles get mass W or Z W or Z H Fermion Fermion

Forces due to Higgs exchange

Next steps to investigate Does it couple to itself? Are there more Higgs bosons? What shape is the Higgs potential? Why is the vacuum energy so close to zero?